DAVID Cameron has made the case for air strikes on Islamic State after Foreign Secretary Philip Hammond warned MPs that Britain has "no choice" but to fight.

David Cameron has told MPs Britain must join US-led air strikes on IS[PA/AP]

The Prime Minister said in an emergency debate on whether Britain should join US-led air strikes on the extremist group's "network of death", after recalling Parliament.

He said there was "no more serious an issue" than deciding whether to commit British forces to the international effort to tackle IS and acknowledged the military effort would last "not just months, but years".

MPs voted today on whether to back the Royal Air Force joining the US-led bombing campaign on IS - also known as Isil - but British air strikes will be limited to Iraq rather than IS strongholds in Syria.

Opening the debate in the Commons Mr Cameron said: "The question before the House today is how we keep the British people safe from the threat posed by Isil and, in particular, what role our armed forces should play in the international coalition to dismantle and ultimately destroy what President Obama has rightly called this network of death.

"There is no more serious an issue than asking our armed forces to put themselves in harm's way to protect our country."

Labour leader Ed Miliband also backed British involvement in the air strikes in a debate made tense by tough questions from MPs on both sides of the house.

He said: "The Prime Minister had said it has ambitions for a state of its own, a caliphate across the Middle Eat run according to their horrific norms and values.

Labour leader Ed Miliband today backed British bombings on IS [PA]

"That's why I believe... we cannot simply stand by against the threat of Isil.

"In acting against them, we need to learn the lessons of the past and we should be clear about this with the British people - that means a comprehensive strategy, humanitarian and political, as well as military, and crucially routed in the region.

"Some of this work is under way but much more needs to be done.

"But there is a reality this House must face up up, which is to make this alliance work there is a need for military action as well to contain and counter the threat of Isil in Iraq. That is why we are meeting today."

Mr Miliband said IS presented reasons to fight a just cause because it threatened Britain's own security by damaging regional stability.

And he said it was clearly not possible or desirable to negotiate with IS.

Mr Miliband added: "I support this motion today because we are responding to the request from the democratic Iraqi state and this is recognised in the UN charter."

Foreign Secretary Philip Hammond said Britian has 'no choice' but to join the fight against IS [GETTY]

Earlier the Foreign Secretary warned that Britain has "no choice" about tackling the threat of Islamic State (IS), as MPs prepare to vote on whether to join air strikes in Iraq.

The minister said the extremist group posed a clear threat to UK national security.

He also reiterated that the bombing campaign could be extended to Syria in future - although he stressed that Parliament would be consulted again before that happened.

The Cabinet unanimously backed military action against IS yesterday, and the Government is expected to secure a large majority in Parliament later today.

Meanwhile, the head of the FBI said it has identified the British extremist known as "Jihadi John" who was shown beheading hostages in three gruesome videos.

Mr Hammond refused to speculate on how long the military campaign against IS could last, after Defence Secretary Michael Fallon suggested it could be up to three years.

Parliament was recalled for today's crucial Commons vote on British action against IS [PA]

"It will take as long as it takes. I don't think anybody can put a timeframe on it," Mr Hammond told Sky News.

"We are going into this with our eyes open. We don't have a choice about engaging with Isil ... it is a threat to Britain's national security, it is a threat to the stability of the region."

Mr Hammond said the Iraqi army needed "restructuring and retraining" after being "damaged" by Nouri Maliki's government.

"But he stressed that the force on the ground was large and "well-equipped", and only required air and intelligence support to defeat IS."

A YouGov poll published in the Sun today suggested that 57 per cent of Britons back British attacks on IS in Iraq, with 51 per cent also supporting action in Syria.

Archbishop of Canterbury Justin Welby today told the house of Lords he backs air strikes in Iraq [GETTY]

Meanwhile, the Archbishop of Canterbury also threw his weight behind proposed British air strikes in Iraq.

The Most Rev Justin Welby told a packed House of Lords: "The action proposed today is right."

But he warned: "We must not rely on a short term solution, on a narrow front, to a global, ideological, religious, holistic and trans-generational challenge.

"We must demonstrate that there is a positive vision far greater and more compelling than the evil of Isil ...

"Such a vision offers us and the world hope - an assurance of success in this struggle - not the endless threat of darkness."

The Archbishop was speaking to a crowded Upper House recalled for an emergency sitting due to the terrorist threat.

His comments came after Lords leader Baroness Stowell of Beeston had condemned the "staggering brutality" of the Islamic State (IS) and insisted Britain had a duty to act alongside its allies.

Unlike MPs, peers will not have a vote on any military action but are debating the issue only on a motion to "take note" of developments in Iraq.